In a production process for automobiles, bores of cylinder blocks of engines are cut, and thereafter, cylinder heads and crank cases are assembled on the cylinder blocks. Since a piston to be placed in a bore is in a shape of a circular cross section, the bore is cut so as to attain a state where its cross section is close to a true circle.
However, even if each bore of the cylinder block is cut into a shape of a circular cross section, when the cylinder head, the crank case and the like are assembled to the cylinder block, the shape of the bore is deformed. Such deformation of the bore may be a factor to increase slide resistance between the bore and the piston caused in use of the engine, and it is apprehended that the engine may not be able to exhibit desired performance.
Therefore, in cutting the bore of the cylinder block, the bore is worked with a dummy head acting like a cylinder head attached to the cylinder block, and after completing the working of the bore, the dummy head is removed. However, when the dummy head or the like is attached and removed every time the bore of the cylinder head is worked, there arises a problem that the productivity is largely degraded.
In order to solve this problem, the following method has been proposed: First, a dummy head is attached to a cylinder head, so as to work a bore into a shape of a circular cross section by using a machining tool.
Next, the dummy head is removed from the cylinder block. Then, stress caused by the dummy head attached thereto is released, and hence, the bore is deformed into a shape of a non-circular cross section. The entire shape of the bore attained after removing the dummy head is measured so as to create NC data. Alternatively, data of the shape of the bore is obtained through simulation. Thereafter, the cylinder block is bored without attaching the dummy head thereto on the basis of the created NC data or the obtained shape data. In this manner, even when a bore is worked without attaching the dummy head to the cylinder block, the bore attains a true circular shape when a cylinder head is attached thereto.
For achieving this method, it is necessary to perform honing of the inside diameter of a bore in a shape of a non-circular cross section after performing the boring in the cylinder block. However, a grind-stone-mount of a conventional honing head used in the honing is made of a metal and is not deformable. Therefore, a cutting (grinding and polishing) amount is large in a portion where the contact pressure of a grind stone against the inside diameter of the bore in a shape of a non-circular cross section is high and the cutting (grinding and polishing) amount is small in a portion where the contact pressure of the grind stone is low. As a result, the shape of the bore attained after completing the honing is unavoidably in a state close to a shape of a circular cross section again from the shape of the non-circular cross section obtained through the boring.
Accordingly, a honing head for keeping the contact pressure of a grind stone substantially constant in the honing has been proposed (see Patent Document 1).